Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase. Thus, a process that uses higher alkenes instead of propylene as feed and co-produces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
One such process proceeds via cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts.
Cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. In this process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce a reaction intermediate such as cyclohexene which then alkylates the benzene starting material. Thus, U.S. Pat. Nos. 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts which comprise nickel and rare earth-treated zeolites and a palladium promoter. Similarly, U.S. Pat. Nos. 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed in these prior art processes are zeolites X and Y. In addition, U.S. Pat. No. 5,053,571 proposes the use of ruthenium and nickel supported on zeolite beta as the aromatic hydroalkylation catalyst. However, these earlier proposals for the hydroalkylation of benzene suffer from the problems that the selectivity to cyclohexylbenzene is low, particularly at economically viable benzene conversion rates, and that large quantities of unwanted by-products are produced.
More recently, U.S. Pat. No. 6,037,513 has disclosed that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting the benzene and hydrogen with a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof, and the contacting step is conducted at a temperature of 50° C. to 350° C., a pressure of 100 kPa to 7000 kPa, a benzene to hydrogen molar ratio of 0.01 to 100 and a weight hourly space velocity (WHSV) of 0.01 hr−1 to 100 hr−1. The '513 patent discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
Although the process of the '513 patent represents a significant improvement over earlier processes for the hydroalkylation of benzene, it still suffers from the problem that significant properties of impurities, particularly cyclohexane, are produced in addition to the desired cyclohexylbenzene. These impurities represent loss of valuable benzene feed and, unless removed, build up in the benzene recycle stream thereby displacing benzene and further increasing the production of undesirable by-products. To address this problem, it has been proposed to contact the cyclohexane with a dehydrogenation catalyst under conditions to selectively convert cyclohexane to benzene, which can then be recycled to the hydroalkylation process. See, for example, U.S. Pat. No. 7,579,511 and International Patent Publication No. WO2009/131769.
Benzene hydroalkylation consumes hydrogen and, although cyclohexane dehydrogenation produces hydrogen, controlling catalyst aging requires that hydrogen is co-fed with the cyclohexane to the dehydrogenation process. Also, the amount of hydrogen consumed in the benzene hydroalkylation process far outweighs the amount of hydrogen produced in the cyclohexane dehydrogenation process. As a result fresh hydrogen must always be introduced in the hydroalkylation/dehydrogenation loop. According to the invention, it has now been found that (1) the life of the hydroalkylation catalyst can be prolonged by contacting the hydrogen stream with the dehydrogenation catalyst prior to the hydroalkylation step; and (2) the selection of the point in this loop where the hydrogen is introduced has a profound effect on efficacy of the entire process, namely the useful life of the catalysts and the amount of benzene recycle can be increased and the formation of certain byproducts, such as bicyclohexane, can be decreased.